I have a problem on my homework that says the potential energy of a particle is given by its position in the x-y plane according to P.E. = x^3 + 8x^2 + 34yz and I have to calculate the force on the particle at point (x,y,z), and all equilibrium points. dU = 3x^2 dx + 16x dx + 34y dz + 34z dy and F = -dU/dr with r being the vector position... -F = 3x^2 dx/dr + 16x dx/dr + 34y dz/dr + 34z dy/dr but that can't be the answer can it? I'm not given any dx/dr's or anything like that...I have a feeling there's some simple math thing I'm missing. Any hints anyone can think of?